It is known that magnesium chloride hexammoniate (MgCl.sub.2 6NH.sub.3) is usable as a material for producing anhydrous magnesium chloride which can be converted into metallic magnesium by using a molten salt electrolysis method. For example, Japanese Pat. No. 89,519 and U.S. Pat. No. 3,092,450 disclose a process for producing anhydrous magnesium chloride by reacting magnesium chloride with ammonia and by decomposing the resultant magnesium chloride hexammoniate. However, the known processes are technically disadvantageous in that the resultant anhydrous magnesium chloride contains a considerable amount of impurities and causes the degree of purity of the metallic magnesium converted therefrom to be poor. Also, the known processes on the recovery of the sensible refrigerant energy and that of unreacted compounds are economically disadvantageous due to the fact that after the reaction for producing magnesium chloride hexammonite at a low temperature, unreacted compounds in the reaction mixture are neither recovered nor reused, and the sensible refrigerant energy of the reaction mixture is discharged without recovery.
For example, in the process of Japanese Pat. No. 89,519, solid magnesium chloride hexahydrate is directly brought into contact with liquid ammonia at a low temperature to produce magnesium chloride hexammoniate by substituting the hexahydrate ligand group in the magnesium chloride hexahydrate by a hexammoniate ligand group. In this ligand group substitution reaction, first, the ammonia comes into contact with the surface of each solid magnesium chloride hexahydrate particle and, then, penetrates into the inside of the particle. The resultant magnesium chloride hexammoniate is insoluble in the liquid ammonia. Therefore, during the ligand group substitution reaction, the peripheral portion of each magnesium chloride hexahydrate particle is replaced by the resultant solid magnesium chloride hexammoniate layer which obstructs the penetration of the liquid ammonia into the inside of the particle. Accordingly, the resultant magnesium chloride hexammoniate particles contain, in their center portions, the unreacted magnesium chloride hexahydrate as an impurity. When the thus produced magnesium chloride hexammoniate is used for producing anhydrous magnesium chloride, the magnesium chloride hexahydrate contained in the hexammoniate causes the resultant anhydrous magnesium chloride to contain a considerable amount of magnesium oxide. Furthermore, in the process of Japanese Pat. No. 89,519, after the magnesium chloride hexammoniate is separated from the reaction mixture, only unreacted ammonia is recovered from the remaining reaction mixture, and the residual liquid which has a low temperature is discharged without recovering the sensible refrigerant energy of the residual liquid. Also, unreacted magnesium chloride hexahydrate in the reaction mixture is neither recovered nor reused.
In the process of U.S. Pat. No. 3,092,450, an aqueous solution of magnesium chloride and, optionally, a soluble ammonium salt such as ammonium chloride, is added to an aqueous solution containing ammonia, at a low temperature, so as to allow the resulting magnesium chloride hexammoniate to precipitate from the reaction mixture, and then, the magnesium chloride hexammoniate is recovered. In this process, since the reaction of the magnesium chloride hexahydrate with the ammonia is carried out in the homogeneous liquid phase, the resultant magnesium chloride hexammoniate has a relatively high degree of purity. However, after the recovery of the precipitated magnesium chloride hexammoniate, the remaining aqueous solution containing unreacted magnesium chloride, ammonium salt and ammonia, and having a low temperature, is discharged from the reaction system without recovering it. That is, in the above-mentioned process, the sensible refrigerant energy of and the unreacted compounds in the remaining aqueous solution are not re-utilized for this process. Furthermore, in order to recover liquid ammonia from the remaining aqueous solution, it is necessary that, first, a mixture of water and ammonia be separated from the remaining aqueous solution and, then, the liquid ammonia be isolated from the mixture. Accordingly, the isolation of the liquid ammonia from the remaining aqueous solution results in the consumption of an extremely large amount of thermal energy.
In connection with the reuse of the remaining aqueous solution containing magnesium chloride, ammonium chloride and ammonia, it should be noted that, when solid magnesium chloride is dissolved in the remaining aqueous solution, the following undesirable reaction may occur. EQU MgCl.sub.2 +NH.sub.4 OH.fwdarw.Mg(OH).sub.2 +NH.sub.4 Cl
That is, this reaction results in undesirable containing of magnesium oxide in the resultant anhydrous magnesium chloride. Accordingly, the reuse of the remaining aqueous solution should be carried out in such a manner that the generation of the magnesium oxide can be avoided.